Fuel inijector and method for controlling fuel injectors

ABSTRACT

A fuel injector for an internal combustion engine, the fuel injector comprising an injector body, a fuel supply passage defined in the injector body, the fuel supply passage containing fuel under high pressure in use of the injector, a pressure sensor for measuring the pressure of fuel in the passage in use, wherein the pressure sensor is situated within the injector body and is separated from fuel in the passage in use, and a method of fuel injection, comprising constructing an hydraulic behaviour profile by fuel pressure measurement, using the hydraulic behaviour profile to predict fuel pressure that will prevail in a fuel injector during an injection event, and supplying a control signal to the fuel injector to control the amount of fuel injected during the injection event in accordance with the predicted fuel pressure. By predicting the fuel pressure that will prevail during an injection event, the fuel delivered during the injection event can be accurately controlled.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.11/890,737, filed Aug. 7, 2007, entitled “FUEL INJECTOR AND METHOD FORCONTROLLING FUEL INJECTORS”.

TECHNICAL FIELD

This invention relates to fuel injectors for internal combustionengines, and methods for controlling the quantity of fuel delivered byfuel injectors. In particular, the invention concerns the determinationor prediction of the pressure of fuel in a fuel injector over the courseof a fuel injection event.

BACKGROUND OF THE INVENTION

Fuel injection systems allow control and optimization of the quantity offuel injected into the combustion chambers of an engine, the timing offuel delivery with respect to the crankshaft and piston position, andthe presentation of fuel to the combustion chamber, for example byatomising and dispersing the fuel in a pre-determined pattern. Modernfuel injection systems use electronic controls to achieve a high levelof precision in the quantity and timing of the fuel delivery. This highprecision is required to meet emissions and performance expectations ofthe marketplace.

Common rail fuel injection systems are well known, particularly in thefield of compression ignition engines such as diesel engines. A typicalcommon rail fuel injection system for an automobile is shownschematically in FIG. 1 of the accompanying drawings. Fuel is stored ina fuel tank 20, and is drawn by way of a lift pump 22 and a filter 24 toan engine-driven high-pressure pump 26. The high-pressure pump 26supplies fuel at elevated pressure to an accumulator or rail 28. Fuelinjectors 30 are connected to the rail by respective jumper pipes 32.Each fuel injector 30 is arranged to supply fuel to one respectivecylinder of the engine by injecting the fuel into a combustion chamberof the cylinder under the control of an electronic control unit (ECU)34.

Many types of fuel injector are known. In a typical arrangement, a fuelinjector includes a control valve comprising a valve needle moveablebetween a first position and a second position upon actuation of anactuator, for example a solenoid or a piezoelectric actuator. The valveneedle is accommodated within a body of the fuel injector. The bodydefines a nozzle provided with at least one orifice downstream of aseating surface for the valve needle. The seating surface, in turn, liesdownstream of a reservoir of fuel at high pressure. In the firstposition, the valve needle seals against the seating surface, so as toprevent flow of fuel past the seating surface. In the second position,the valve needle is held away from the seating surface, so that fuel canflow from the reservoir, through the or each orifice and into thecombustion chamber, thus effecting an injection of fuel.

The quantity of fuel delivered to the combustion chambers affects thetorque output of the engine. Consequently, fuel delivery must becarefully controlled to provide the desired torque output at any giventime under the conditions then prevailing.

The quantity of fuel delivered over the course of each injection eventis a function of the nozzle orifice flow area, the fuel pressure and theinjection duration. The injection duration is the time over which theneedle is lifted from the seating surface, so that high-pressure fuelcan flow into the combustion chamber through the orifice.

In a given fuel injector, the nozzle orifice flow area is fixed. Fueldelivery is therefore controlled using the so-called ‘pressure-time’principle. To achieve delivery of a desired quantity of fuel, theinjection duration is set electronically to a value which has beenpre-calculated so that, assuming a certain fuel pressure, the requiredquantity of fuel will pass into the combustion chamber over the timethat the fuel can flow through the nozzle, i.e. the injection duration.Consequently, any unintended variation in the fuel pressure may resultin an incorrect quantity of fuel being delivered to the combustionchamber, with the result that the engine produces an output torque whichis more or less than required. In these circumstances, the driveability,performance and emissions of the vehicle may be compromised.

Referring again to FIG. 1, control of the injection timing and durationis achieved by the ECU 34. The ECU 34 accepts input signals from avariety of sensors, which may include a crankshaft speed sensor 36 a, acrankshaft phase sensor 36 b, a throttle pedal demand sensor 36 c, anair intake temperature sensor 36 d, a coolant temperature sensor 36 e,an air intake mass flow sensor 36 f, and, in turbocharged engines, anintake boost pressure sensor 36 g. In addition, common rail fuelinjection systems include a fuel rail pressure sensor 38, which may becombined with a fuel temperature sensor. The ECU 34 controls, by way ofoutput signals, various actuators which actuate a metering flow valve 40at the inlet of the high-pressure pump 26, a rail pressure control valve42, and control valves of the individual injectors 30.

The rail pressure sensor 38 is typically a piezo-resistive device withintegrated electronics. It is installed intrusively in the rail 28, sothat a portion of the sensor body, typically a diaphragm, is directlyexposed to the high-pressure fuel in the rail 28. Generally, the railpressure sensor 38 is screwed into a threaded port 44 in the rail 28,and a soft iron washer may be used to effect a seal between the sensor38 and the rail 28. As rail pressure sensors 38 must operate reliablyand without leakage in a very high-pressure environment, such sensors 38are relatively expensive and delicate.

The nominal fuel pressure in the rail 28, and hence in each fuelinjector 30, is determined by the ECU 34 using the input signals fromthe sensors 36 a-36 g, 38 to determine the engine operating conditionsand the torque requirement. For example, at low engine speeds and lowloads, the nominal rail pressure may be 300 bar; while at high enginespeeds and high loads the nominal rail pressure may be 2000 bar.Typically, a range of optimum nominal rail pressures is recorded for acorresponding range of conditions in a calibration procedure duringengine set-up and testing. The optimised values are determined so as tominimise emissions, optimise performance, or minimise fuel consumptionas required. These optimised nominal pressures are stored in a map in amemory of the ECU 34 so that the optimised value for a given enginecondition can be retrieved.

Under a given set of engine conditions, therefore, the nominal mean railfuel pressure has a fixed value. The ECU 34 determines the actual,instantaneous rail fuel pressure from the rail pressure sensor 38, andoperates the inlet metering flow valve 40 of the high-pressure fuel pump26 or the rail pressure control valve 42 as appropriate to achieve andmaintain the desired mean rail fuel pressure. In this way, a feedbackcontrol system is provided. Sophisticated control algorithms areprovided to optimise this feedback control system. It is important thatthe rail pressure sensor is as accurate as possible because unexpectedvariations in rail fuel pressure will cause unexpected variations intorque output.

The response time of the feedback control system is limited by theperformance of the rail pressure sensor 38, the ECU 34, thehigh-pressure pump 26 and the inlet metering flow valve 40 or the railpressure control valve 42. For example, if the rail pressure drops, therail pressure sensor 38 must respond to the pressure drop by sending anappropriate signal to the ECU 34, the ECU 34 must then evaluate thesignal and respond by actuating the inlet metering flow valve 40, andwithin the constraints of its flow capacity, the high-pressure pump 26must increase the rail pressure to the required value.

An injection event places an instantaneous flow demand on the fuelvolume stored in the rail 28. The instantaneous flow demand is such thatthe control system cannot respond rapidly enough and, as a consequence,the fuel pressure in the rail 28 drops. The fuel pressure in the rail 28is therefore perturbed, and a short time elapses before the pressurerecovers to the desired level, although this recovery is hopefullycomplete before the next injection event. The drop in pressure meansthat, over the duration of a normal injection event, the mean pressurein the rail 28 may be slightly below the target pressure, but thiseffect can be accounted for during calibration so that the anticipatedtorque is still achieved.

Recent developments in fuel injection technology, and of common railsystems in particular, have introduced the capability of delivering fuelin multiple injection events per combustion cycle. In other words,instead of a single injection event occurring during each cycle of thecylinder, the fuel is delivered in a sequence, or train, of two or moreprecisely timed injection events, each of which injects a carefullycontrolled quantity of fuel. For example, an injection sequence maycomprise a pilot injection or pre-injection, which pre-heats the gasesin the combustion chamber ahead of a main injection in which themajority of the fuel is injected. A post injection, after the maininjection, may also be provided to encourage complete combustion ofunburnt fuel, thus reducing harmful exhaust emissions and improving fuelefficiency.

Modern engines, therefore, may utilise multiple injection events percycle to optimise performance and fuel efficiency and to reduce harmfulexhaust emissions. Over a range of engine load and speed conditions, theoptimum injection sequence may change. For example, some conditions mayrequire a pilot injection closely followed by a main injection, someconditions may require a split main injection, other conditions mayrequire pilot, main and post injections, while still other conditionsmay require multiple pilot or multiple post injections.

When sequences of multiple injection events are required, thepossibility arises that a perturbation to the rail pressure, and henceto the fuel pressure in the injectors 30, caused by a prior injectionevent may still be present when a subsequent injection event begins. Inother words, the pressure wave within the fuel system that results fromthe prior injection event may not have died away when the subsequentinjection event occurs. Consequently, the fuel pressure in the injector30 at the time of the subsequent injection event is not at the expectedlevel, corresponding to the target rail pressure. Instead, the pressurein the injector 30 is lower or higher than the expected pressure,depending on the phase relationship of the pressure wave to thesubsequent injection event. In either case, the result is that anincorrect, unpredicted and unpredictable quantity of fuel is delivered,with similarly unpredictable consequences for torque output andemissions.

Significant errors in the fuel quantity delivered can arise because ofthis phenomenon, and these errors can result in unacceptable emissions,increased noise, impaired driveability, poor performance and so on.

One known approach to reduce or mitigate the undesired effects of theseresidual pressure waves involves providing tuning orifices at particularlocations in the fuel system to damp the pressure waves resulting frominjection events, thus impeding propagation of the waves. However, thisapproach is inflexible because the tuning orifices are effective onlyover a relatively limited range of engine conditions and injectionsequences. In particular, this approach is of limited value where morethan one injection strategy is employed in a given engine.

The effects of pressure waves in multiple-injection sequences could, intheory, be compensated for by mapping the entire speed and load regimeof the engine with fine granularity and calibrating the injectiondurations in the sequence to compensate for residual pressure waves.However, this approach is impractical because it would require anextremely laborious calibration procedure, as well as the storage andrapid retrieval of a huge amount of data by the ECU. Furthermore, thecalibrated injection durations would be sensitive to minor changes inpipe lengths and build tolerances.

It is against this background that the present invention has beendevised.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda fuel injector for an internal combustion engine, the fuel injectorcomprising an injector body, a fuel supply passage defined in theinjector body, the fuel supply passage containing fuel under highpressure in use of the injector, and a pressure sensor for measuring thepressure of fuel in the passage in use, wherein the pressure sensor issituated within the injector body and is separated from fuel in thepassage in use. For example, the supply passage may be at leastpartially defined by a wall of the injector body, so that the pressuresensor is separated from the fuel in the passage by the wall.

The pressure sensor may measure a strain experienced by the injectorbody, which arises from and relates to the pressure of fuel in thepassage in use of the injector. In this context, the pressure sensor maycomprise a strain gauge. It is also possible that the pressure sensor isresponsive to a displacement, deflection or deformation of all or partof the injector body, caused by and corresponding to the pressure withinthe fuel supply passage.

Unlike a conventional ‘intrusive’ pressure sensor, such as a railpressure sensor, the pressure sensor provided in the fuel injector ofthe present invention does not intrude into the passage, and no part ofthe pressure sensor need be wetted by the fuel within the passage. Thusthe pressure sensor provided in the invention can be considered a‘non-intrusive’ or ‘dry’ pressure sensor.

The present invention also offers advantages when compared to providingintrusive pressure sensors in individual injectors. According to theinvention, the pressure sensor is separated from the supply passage, sothere is no need for a precision-machined bore or breach in the supplypassage within the injector. Such a bore would be expensive to fabricateand would provide a site for potential leakage or mechanical failure.Furthermore, the pressure sensor is not subject to the high-pressureenvironment of the supply passage. This reduces the risk of mechanicalfailure of the sensor, and because the sensor need not be engineered toresist high pressures, the cost of the sensor can be minimised.Similarly, because the sensor need not be leak-proof, the design of thepressure sensor can be relatively straightforward, resulting in a robustand low cost device. For example, high-pressure seals are not required.

The injector body may define a pressure sensor cavity to accommodate thepressure sensor, the pressure sensor cavity being separated from thepassage. In one variant, the pressure sensor cavity accommodateselectrical connections for an actuator of the injector. In this way, thepressure sensor and its associated electrical connections or harness canbe integrated into an electrical connector, or header, for the actuator.The injector body may further define an actuator cavity and the pressuresensor cavity may communicate with the actuator cavity.

The pressure sensor cavity may extend inwardly from a side of theinjector or inwardly from an end of the injector. The pressure sensorcavity may accommodate an electronic module in electrical communicationwith the pressure sensor.

The pressure sensor may be provided in any suitable location within theinjector body. In one arrangement, the fuel supply passage and thepressure sensor define respective central longitudinal axes thatsubstantially intersect. In another example, the fuel supply passageincludes a portion of enlarged cross-sectional area and the pressuresensor is aligned with that portion of the fuel supply passage.

In one embodiment, the injector body defines an outer wall and an innerwall opposed to the outer wall about the fuel supply passage, and thepressure sensor is separated from fuel in the passage in use by theinner wall. Optionally, a face of the pressure sensor extends parallelto a wall of the supply passage.

The injector may be elongate to define a longitudinal axis, and thepressure sensor may cooperate with a wall of the supply passage thatextends substantially parallel to the longitudinal axis of the injector.Alternatively, an elongate injector may include a pressure sensor whichcooperates with a wall of the supply passage that extends across thelongitudinal axis of the injector.

To optimise the response of the pressure sensor, a face of the pressuresensor may abut a wall which defines the supply passage. To this end,the injector may comprise a clamping element to press the face againstthe wall. For example, the injector body may define a port and theclamping element may be a plug in threaded engagement with the injectorbody within the port.

When a clamping element is provided, electrical connections for thepressure sensor may be accommodated within the clamping element.Similarly, an electronic module in electrical communication with thepressure sensor may be accommodated within the clamping element.

Any suitable pressure sensor may be used. The pressure sensor may, forexample, comprise a magnetostrictive pressure sensor having a core ofmagnetostrictive material. In one embodiment, the pressure sensorcomprises a core being a body of revolution with a generally I-shapedcross-section, and the core may be magnetostrictive. The pressure sensormay comprise a core that is integral with the injector body, in whichcase the pressure sensor may be responsive to changes in strain withinthe core.

In another aspect, the invention extends to a method of calibrating afuel injector according to the first aspect of the invention and havinga pressure sensor, the method comprising measuring an output value ofthe pressure sensor, and determining the pressure of a fluid in thesupply passage corresponding to the output value. The fluid may, forexample, be a gas such as air, or a liquid such as oil or fuel. Themethod may include measuring a plurality of output values of thepressure sensor, and determining, for each one of the plurality ofoutput values, a corresponding pressure of fluid in the supply passage.In this way, a calibration curve for the pressure sensor can beobtained.

The injector may be calibrated after manufacture but before installationin an engine. For calibration purposes, the supply passage may besupplied with fluid at a known pressure or in a sequence of knownpressures. The pressure of fluid corresponding to the output value as asensor characteristic of the injector may be recorded, and the sensorcharacteristic may be encoded, for example in a machine-readable dataformat such as a barcode, or as an alphanumeric code for reading by ahuman operator.

The invention also extends to a method of programming a control unit ofan engine including a fuel injector according to the first aspect of theinvention and having a pressure sensor, the method comprisingcalibrating the fuel injector as previously described, and inputting therecorded sensor characteristic to the control unit, for example byreading an encoded sensor characteristic.

In another aspect, the invention resides in a fuel injection system foran internal combustion engine, the system comprising a plurality of fuelinjectors, each in accordance with the first aspect of the invention,and having a pressure sensor, an accumulator arranged to supply fuel tothe fuel injectors in use and a control unit arranged to receivepressure signals from the pressure sensors of the fuel injectors and tosupply control signals to the fuel injectors to control the injection offuel.

A rail fuel pressure sensor need not be provided, so avoiding the costsof the rail pressure sensor itself, and of providing aprecision-machined bore in the fuel rail for mounting the rail pressuresensor. Furthermore, by eliminating the breach in the fuel rail requiredto accept the sensor, the risk of fuel leakage and mechanical failure atthis site is avoided.

The fuel injection system may include a pump for pressurizing fuel inthe accumulator, and an accumulator pressure control valve forcontrolling fuel pressure in the accumulator. Both the pump and thepressure control valve are under the control of the control unit. Inthis way, the pressure sensor, the pump, the pressure control valve andthe control unit can, in combination, control the pressure of fuel inthe accumulator.

By providing a fuel pressure sensor within each injector, measuredvalues of the fuel pressure in each injector can be obtained and inputto the control unit or ECU. The ECU can use these measured injector fuelpressure values to calculate the injection duration for each injectionevent, rather than relying on an estimated injector fuel pressure basedon measured values of the rail fuel pressure remote from the injector asin conventional systems. In this way, the quantity of fuel delivered inan injection event can be more accurately predicted and controlled.

Accordingly, the control unit may be arranged to receive pressuresignals from the pressure sensors to construct an hydraulic behaviourprofile, to predict, using the hydraulic behaviour profile, the fuelpressures that will prevail in the injectors during injection events,and to supply control signals to the fuel injectors to control theamount of fuel injected during those injection events in accordance withthe predicted fuel pressures.

To this end, the control unit may comprise a processor programmedrepeatedly to sample the pressure signals from the pressure sensors toconstruct the hydraulic behaviour profile. In one variant, the controlunit comprises a memory for storing an hydraulic behaviour model and aprocessor programmed to apply the hydraulic behaviour profile to thestored model to predict the fuel pressures that will prevail in theinjectors during injection events.

In another aspect, the present invention resides in a method of fuelinjection comprising constructing an hydraulic behaviour profile by fuelpressure measurement, using the hydraulic behaviour profile to predictfuel pressure that will prevail in a fuel injector during an injectionevent, and supplying a control signal to the fuel injector to controlthe amount of fuel injected during the injection event in accordancewith the predicted fuel pressure.

By predicting the fuel pressure that will prevail in a fuel injectorover the course of a forthcoming injection event in this way, the amountof fuel injected during the injection event can be more accuratelycontrolled than would otherwise be possible, and accurate control can beachieved for all engine operating conditions and injection strategies.

Optionally, the hydraulic behaviour profile is constructed by repeatedlysampling fuel pressure. The method may include retrieving a storedhydraulic behaviour model, and applying the hydraulic behaviour profileto the stored model to predict the fuel pressures that will prevail inthe fuel injector during the injection event.

One variant of the method includes constructing an hydraulic behaviourprofile by measuring fuel pressure in an accumulator arranged to supplyfuel to a plurality of fuel injectors in use, predicting, using thehydraulic behaviour profile, the fuel pressures that will prevail in thefuel injectors of the plurality during injection events, and controllingthe fuel injectors of the plurality to control the amount of fuelinjected during injection events in accordance with the predicted fuelpressures.

Accordingly, in another aspect, the invention resides in a fuelinjection system for an internal combustion engine, the systemcomprising a plurality of fuel injectors, an accumulator arranged tosupply fuel to the fuel injectors in use, a pressure sensor formeasuring the pressure of fuel in the accumulator in use, and a controlunit arranged to receive a pressure signal from the pressure sensor toconstruct an hydraulic behaviour profile, to predict, using thehydraulic behaviour profile, the fuel pressure that will prevail in aninjector during an injection event, and to supply control signals tothat fuel injector to control the amount of fuel injected during thatinjection event in accordance with the predicted fuel pressure.

The control unit may comprise a processor programmed repeatedly tosample the pressure signal from the pressure sensor to construct thehydraulic behaviour profile. Alternatively, or in addition, the controlunit may comprise a memory for storing an hydraulic behaviour model anda processor programmed to apply the hydraulic behaviour profile to thestored model to predict the fuel pressure that will prevail in theinjectors during injection events.

In another aspect, the invention resides in a method of predicting afuel pressure characteristic in a fuel injector of a fuel injectionsystem during an injection event, the method comprising measuring,before the injection event, a fuel pressure characteristic within thefuel injection system, and determining, using the measured fuel pressurecharacteristic, a predicted fuel pressure characteristic in the fuelinjector during the injection event. Again, the fuel pressurecharacteristic may be measured by repeatedly interrogating a pressuresensor before the injection event, and the measured fuel pressurecharacteristic may comprise a sequence of fuel pressure values.

The fuel pressure characteristic may be measured within the fuelinjector, for example when the fuel injectors of the fuel injectionsystem are constructed according to the first aspect of the invention.Alternatively, the fuel pressure characteristic may be measured at alocation in the fuel injection system upstream of the fuel injector, forexample in an accumulator upstream of the fuel injector.

In one variant of the method, the measured fuel pressure characteristicis input to a model for hydraulic behaviour to determine the predictedfuel pressure characteristic in the fuel injector during the injectionevent. The predicted fuel pressure characteristic may comprise apredicted average fuel pressure in the fuel injector during theinjection event.

By virtue of this aspect of the invention, the predicted fuel pressurecharacteristic can, for example, be used within the ‘pressure-time’principle to determine an accurate value for the duration of aninjection event required in order to deliver a desired quantity of fuel.To this end, the invention extends to a method of correcting a nominalfuel injection demand, comprising predicting a fuel pressurecharacteristic as described above, calculating a correction factor basedon the predicted fuel pressure characteristic during the injectionevent, and applying the correction factor to the nominal fuel injectiondemand to compensate for variations in fuel pressure during theinjection event. The nominal fuel injection demand may, for example,comprise the duration of a fuel injection event that would be requiredfor delivery of a desired amount of fuel if the fuel pressurecharacteristic in the fuel injector were a constant pressure equal tothe nominal pressure of fuel in an accumulator upstream of the injector.

Accordingly, the invention further extends to determining a nominal fuelinjection demand according to engine operating parameters, andcorrecting the nominal fuel injection demand in accordance with themethod described above.

When pressure sensors are provided in the fuel injectors, the local fuelpressures measured by the pressure sensors can be used to estimate thepressure of fuel in an accumulator, and thus provide an input to afeedback loop to control the fuel pressure in the accumulator. Thus, inanother aspect, the invention resides in a method of estimating thepressure of fuel in an accumulator of a fuel injection system, themethod comprising measuring local fuel pressures in a plurality of fuelinjectors connected to the accumulator, and calculating an average valueof the measured local fuel pressures to estimate the fuel pressure inthe accumulator.

Any suitable strategy for calculating the average value of the measuredlocal fuel pressures may be used. For example, the method may includeweighting the measured local fuel pressures to account for the hydrauliccharacteristics of the fuel injection system.

A degree of robustness against sensor failure can be provided by virtueof another aspect of the present invention, in which a method ofestimating the pressure of fuel in an accumulator of a fuel injectionsystem is contemplated, the method comprising measuring local fuelpressures in a plurality of fuel injectors connected to the accumulator,detecting erroneous pressure signals from one injector of the plurality,and excluding the erroneous pressure signals from that injector fromcalculation of the pressure of fuel in the accumulator. Thus, if thepressure sensor of an injector fails, an estimate of the pressure offuel in the accumulator can still be obtained.

In another aspect of the invention, there is provided a method ofcompensating for an error in the output of a fuel pressure sensor, themethod comprising determining an average output of the fuel pressuresensor, comparing the average output of the fuel pressure sensor with areference value, and, if the average output of the fuel pressure sensordiffers from the reference value by more than a first pre-determinedthreshold value, applying a compensation offset to the output of thefuel pressure sensor.

In this way, sensor drift, offset inaccuracies, calibration errors andother such systematic variations in sensor output can be corrected orcompensated for.

The method may include retrieving a stored compensation offset, forexample a pre-determined value for incremental correction of the errorin the output of the fuel sensor. In another variant, the methodincludes calculating a difference between the average output of the fuelpressure sensor and the reference value, and applying the difference tothe output of the fuel pressure sensor as the compensation offset. Inthis case, the method may further comprise storing the compensationoffset.

The method of compensating for an error in the output of a fuel pressuresensor may include an adaptive or incremental strategy for compensatingfor the error. In one such example, the method includes determining,after the compensation offset has been applied, an average output of thefuel sensor, comparing the average output of the fuel pressure sensorwith the reference value, and, if the average output of the fuelpressure sensor differs from the reference value by more than a secondpre-determined threshold value, applying a further compensation offsetto the output of the fuel pressure sensor.

The method may include retrieving a stored reference value. For example,the reference value may be a pre-determined value stored in a memory. Ina variant of the method, the reference value is calculated using anaverage output of one or more further fuel pressure sensors.

Again, a degree of robustness against sensor failure can be provided inaccordance with another aspect of the invention, which resides in amethod of predicting a fuel pressure characteristic in a fuel injectorof a fuel injection system during an injection event, the methodcomprising measuring, before the injection event, fuel pressurecharacteristics within a plurality of fuel injectors, detecting anerroneous pressure signal from one injector of the plurality, andexcluding the erroneous pressure signal from prediction of the fuelpressure characteristic.

So that a reasonable prediction of the fuel characteristic can be made,the method may comprise deriving a predicted fuel pressurecharacteristic from other injectors of the plurality, and applying thatcharacteristic to the injector from which the erroneous pressure signalwas detected.

In further aspects, the invention extends to a computer program productcomprising at least one computer program software portion which, whenexecuted in an execution environment, is operable to implement any ofthe methods according to the invention described above, and a datastorage medium having the or each such computer program software portionstored thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the accompanying drawings, which is a schematic diagram of aknown common-rail fuel injection system, has already been discussed.Preferred embodiments of the present invention will now be described, byway of example only, with reference to the remaining drawings in whichlike reference numerals are used for like features, and in which:

FIG. 2 is a side view of a first embodiment of a fuel injector accordingto the present invention;

FIG. 3 is an enlarged longitudinal section of part of the fuel injectorof FIG. 2;

FIG. 4 is a partial longitudinal section of part of a second embodimentof a fuel injector according to the present invention;

FIG. 5 is a cross-section through the fuel injector of FIG. 4;

FIG. 6 is a longitudinal section of part of a third embodiment of a fuelinjector according to the present invention;

FIG. 7 is a schematic diagram of a common rail fuel injection systemaccording to the present invention that may incorporate any of the fuelinjectors shown in FIGS. 2 to 6;

FIG. 8 is a flowchart of a method of calibrating a fuel injectoraccording to the present invention;

FIG. 9 is a simplified schematic diagram of apparatus arranged toperform the method of FIG. 8;

FIG. 10 is a flowchart of a method of programming an ECU with pressuresensor calibration information;

FIG. 11 is a simplified schematic diagram of apparatus arranged toperform the method of FIG. 10;

FIG. 12 is a flowchart of a method for compensating for an error in theoutput of a fuel pressure sensor;

FIG. 13 is a flowchart of a first expression of a method of fuelinjection according to the present invention;

FIG. 14 is a simplified schematic diagram of elements of the fuelinjection system of FIG. 7, particularly an ECU and fuel injectors,arranged to perform the method of FIG. 13;

FIG. 15 is a schematic diagram showing the evolution of fuel pressureover time expressed as a fuel pressure sensor output (vertical axis)against time (horizontal axis) within a fuel injector;

FIG. 16 is a flowchart of a method of predicting a fuel pressurecharacteristic in a fuel injector in the case when an erroneous signalis detected;

FIG. 17 is a flowchart of an alternative method of fuel injectionaccording to the present invention; and

FIG. 18 is a schematic diagram of elements of a fuel injection system,further including a fuel rail fitted with a fuel rail pressure sensor,arranged to perform the method of FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2 and 3 show an exemplary fuel injector 46 according to a firstembodiment of the present invention. FIG. 2 is a side view of theelongate injector 46 showing its longitudinal axis 48. FIG. 3 is anenlarged detailed view of the injector 46 in longitudinal section takenparallel to the longitudinal axis 48 of the injector 46.

The injector 46 comprises a generally cylindrical injector body 50which, in use, extends through a cylinder head of an internal combustionengine. The upper part of the injector body 50 is provided with a fuelinlet 52, which receives high-pressure fuel from a common fuel rail (notshown). The lower part of the injector body 50 comprises a nozzle 54arranged to inject fuel into a combustion chamber of the engine.

As is well known in the art of fuel injectors, the nozzle 54 houses avalve needle moveable between two positions. In a first, closedposition, the needle seals against a seating surface of the nozzle 54 toprevent the flow of fuel through one or more orifices 56 provided in thenozzle 54. In a second, open position, the valve needle is retractedfrom the seating surface so as to allow fuel to flow through the or eachorifice 54. The valve needle may be of the inwardly or outwardly openingtype. The valve needle and the seating surface have been omitted fromthe drawings but may be arranged as shown in the Assignee's U.S. Pat.Nos. 6,234,404 and 7,159,799, the contents of which are incorporatedherein by reference.

The central part of the injector body 50 houses an actuator for controlof the valve needle. The actuator may, for example, be a piezoelectricactuator or a solenoid actuator. Part or all of the actuator undergoeslongitudinal strain or is displaced longitudinally when the actuator isenergised by the application of an electrical signal to electrodes ofthe actuator. Again, such actuators are well known in the art includingthe Assignee's US patents identified above, and so have been omittedfrom the drawings.

A coupling is provided between the actuator and the valve needle, sothat strain or displacement of the actuator, achieved by energising orde-energising the actuator, causes opening or closing movement of theneedle. The coupling may, for example, comprise a mechanical connectionbetween the actuator and the valve needle. The coupling may insteadcomprise a hydraulic coupling, in which case the operation of theactuator causes a pressure change in a chamber associated with the valveneedle to provide an opening or closing force to the needle.

The actuator is accommodated within a chamber 58 in the injector body50. The electrical connections to the actuator, which are typically inthe form of blade terminals, are located within a further chamber orterminal cavity 60. A port 62 extends from the outside surface of theinjector 46 into the injector body 50 to connect with the terminalcavity 60 and afford access to the electrical connections.

In this first embodiment of the invention, the injector 46 is providedwith a pressure sensor 64 which is accommodated within the terminalcavity 60. Electrical connections to the pressure sensor 64 are made byway of the port 62, so that the electrical connections for the actuatorand the pressure sensor 64 can be provided in a single connector (notshown) that fits into the terminal cavity 60. The electrical connectionsallow signals to be transmitted from the pressure sensor 64 to an ECU.The pressure sensor 64 may include electronic circuitry for conditioningthe output signals of the pressure sensor 64.

A conduit or supply passage 66 is provided in the injector body 50 toallow the passage of high-pressure fuel from the fuel inlet 52 to thenozzle 54. The supply passage 66 has a wall 68 integral with theinjector body 50, which separates the supply passage 66 from otherpassages and cavities within the injector body 50, for example theterminal cavity 60. Thus, the wall 68 retains high-pressure fuel withinthe supply passage 66 in use of the injector 46.

A return or back-leak passage 70 is also provided within the injectorbody 50 to return excess fuel to the fuel tank or other reservoir. Aback-leak or return port 72 is provided in the upper part of theinjector body 50 to allow connection of a fuel return pipe to theinjector 46.

In use, the supply passage 66 is filled with fuel from the rail at highpressure via the fuel inlet 52. The pressure within the supply passage66 gives rise to a strain within the injector body 50. Changes in fuelpressure within the supply passage 66 cause corresponding changes in thestrain in the injector body 50. The pressure sensor 64 is arranged torespond to changes in the strain within the injector body 50, and inparticular in a portion of the wall 68 that separates the pressuresensor 64 from the supply passage 66. In this way, the pressure sensor64 provides an output signal that corresponds to the pressure of fuel inthe supply passage 66.

Because the pressure sensor 64 is separated from the supply passage 66by the wall 68, the pressure sensor 64 is not wetted by the fuel. Inthis way, the pressure sensor 64 is not directly subjected to thehigh-pressure environment within the supply passage 66, and need not beleak-proof. Furthermore, the supply passage 66 is not breached by a portor conduit, which would be required if the pressure sensor were to be indirect contact with the fuel in the supply passage 66. The risk ofleakage or failure associated by such a breach in the supply passage 66is therefore avoided.

FIG. 4 is a partial longitudinal section of part of a fuel injector 74according to a second embodiment of the invention, and FIG. 5 is across-section through the injector 74 of FIG. 4 taken normal to thelongitudinal direction. The construction of the fuel injector 74 of thesecond embodiment is similar to that of the first embodiment, and onlythe differences will be described in detail.

In this embodiment, the pressure sensor is not located within a terminalcavity of the injector body. Instead, the injector body 76 is providedwith a sensor chamber 78 close to the fuel inlet 52, and a port 80 whichleads from the sensor chamber 78 to the outside surface of the injector74. A magnetostrictive pressure sensor 82 is located within the sensorchamber 78.

As shown most clearly in FIG. 5, the pressure sensor 82 is separatedfrom the high-pressure fuel supply passage 66 by a wall 68 formed withinthe injector body 76. In this way, the pressure sensor 82 is influencedby strain in the injector body 76 in the region of the supply passage66. Although the pressure sensor 82 is also located close to theback-leak passage 70, the fuel pressure within the back-leak passage 70is low and does not result in any significant strain within the injectorbody 76.

The sensor 82 includes a slug or core 84 of magnetostrictive material.The general shape of the core 84 is a cylinder of revolution with an‘I’-shaped cross-section, as shown most clearly in FIG. 4. The corecomprises a proximal end face 86, closest to the port 80, and a distalend face 88 closest to the supply passage 66 and abutting the wall 68.Thus, the distal end face 88 of the core 84 abuts the end of the sensorchamber 78 closest to the supply passage 66. The cylinder axis of thecore 84 lies normal to the longitudinal axis of the injector 74.Furthermore, the cylinder axis of the core 84 lies normal to thelongitudinal direction of the supply passage 66, so that the distal endface 88 of the core 84 lies parallel to the sensor passage 66.

A coil 90 is wound around the narrow part or neck of the I-section core84. At its proximal end, the core 84 is provided with a groove 92 havingan inclined first portion 94 and a second portion 96 which lies parallelto the cylinder axis of the core 84. One end of the inclined portion 94of the groove 92 intersects the narrow part of the core 84, and thegroove 92 extends to the proximal end face 86 of the core 84. Theproximal end face 86 is provided with a central land or projection 98,and the second portion 96 of the groove 92 extends within the projection98 to define a ‘U’-shaped channel. Connecting wires 100 from the coil 90are routed from the core 84 to the port 80 by way of the groove 92.

Electrical connections, in the form of terminal sockets 102, aredisposed within the port 80, so that the sensor 82 can be connected tothe ECU of an engine by way of a suitable connector (not shown). Theterminal sockets 102 are connected to the connecting wires 100 and aresupported within a clamp screw 104 by an insulating material 106, suchas a ceramic material. The clamp screw 104 comprises an annular plugwhich carries external threads to mate with internal threads provided inthe port 80.

The clamp screw 104 exerts an axial force on the core 84 of the sensor82, so that the distal end face 88 of the core 84 is pressed firmlyagainst the end of the sensor chamber 78. In this way, the strain in theinjector body 76 adjacent to the sensor 82, and in particular the strainin the wall 68, resulting from and corresponding to the fuel pressure inthe supply passage 66, causes deformation of the core 84 of the sensor82.

The magnetic permeability of the core 84 changes in response to anapplied stress. Thus, when the strain in the injector body 76 changes,corresponding deformation of the core 84 of the sensor 82 causes achange in its magnetic permeability. By measuring the inductance of thecoil 90, via the electrical connections 102, the change in strain in theinjector body 76, and in particular the strain in the wall 68, whicharises from and relates to the fuel pressure in the supply passage 66can be detected.

FIG. 6 shows, in longitudinal section, part of a fuel injector 108according to a third embodiment of the invention. The third embodimentof the invention is similar to the first and second embodiments, exceptin the arrangement of the fuel supply passage, the inlet port, and thepressure sensor.

FIG. 6 shows only the uppermost part of the fuel injector 108, opposedto the nozzle. The supply passage 110 extends within the injector body112 and comprises a longitudinal portion 114 and an inclined portion116. The two portions 114, 116 meet at an elbow 118. The longitudinalportion 114 of the supply passage 110 extends from the elbow 118 towardsthe nozzle (not shown). The inclined portion 116 of the passage 110extends from the elbow 118, across the width of the injector 108, to theouter side surface of the injector 108 to form an inlet port 120. Unlikethe first and second embodiments of the invention, in this thirdembodiment the inlet port 120 is provided on the side of the injector108, rather than at the top.

A sensor chamber 122 is provided within the injector body 112, above theinclined portion 116 of the supply passage 110. A threaded port 124connects the sensor chamber 122 to the uppermost, top surface of theinjector 108. As in the second embodiment of the invention, amagnetostrictive pressure sensor 82 comprising a core 84 and a coil 90is provided in the sensor chamber 122.

The distal end face 88 of the core 84 is located close to the inclinedportion 116 of the fuel supply passage 110, in a region 126 where theinclined portion 116 has an enlarged diameter. The enlarged diameterregion 126 may, for example, accommodate a filter or a flow-conditioningdevice (not shown). The pressure sensor 82 is inclined to thelongitudinal direction of the injector 108, so that the distal end face88 of the core 84 lies parallel to the side wall of the enlargeddiameter region 126 closest to the sensor 82.

The pressure sensor 82 is held in position by a clamp screw 128 locatedin the port 124. Like the preceding embodiment, terminal sockets 102 areprovided in a central portion of the clamp screw 128, within a plug 106of insulating material. In this embodiment, the clamp screw 128 has atubular forward extension that bears against the proximal end face 86 ofthe core 84 to provide a clamping force on the core 84. A cylindricalspacer 130 is provided within the tubular forward extension between theproximal end face 86 of the core 84 and the insulating plug 106, andconnecting wires 132 from the coil 90 pass through the spacer 130 to theterminal sockets 102. The spacer 130 may house an electronic module, soas to provide signal conditioning electronics for the pressure sensor82.

As in the second embodiment of the invention, the pressure sensor 82 isresponsive to changes in strain in the body 112 of the injector 108, andparticularly changes in strain in the wall 68 of the supply passage 110,which result from changes in fuel pressure in the supply passage 110.

For a given fuel pressure, the strain in the body 112 of the injector108 is larger close to the enlarged diameter region 126 of the supplypassage 110 than close to regions of the supply passage 110 where thediameter of the passage 110 is not enlarged. Thus, by positioning thesensor 82 close to the enlarged diameter region 126, the response of thepressure sensor 82 is optimised.

Many modifications to and variations of the fuel injector of theinvention are possible. Some such modifications will now be described,by way of example only.

The pressure sensor provided in the injector body may be of any suitabletype. For example, a magnetostrictive sensor corresponding to any of thetypes described in the present applicant's U.S. Pat. Nos. 7,234,361 and7,146,866, or in the present applicant's United States PatentApplication Publication No. 2006/0016277, may be provided. The contentsof those documents are hereby incorporated by reference. It isconceivable that the core of the sensor could be integrated with theinjector body.

Instead of a magnetostrictive sensor as described above, a piezoelectricor piezoresistive pressure sensor could be used. The pressure sensor mayproduce an output signal which relates to the magnitude of the staticstrain in the injector body. This would be the case when the pressuresensor comprises a piezoresistive strain gauge. Instead, the output ofthe pressure sensor may relate to only dynamic changes in the strain,for example when the pressure sensor comprises a piezoelectric straingauge.

Temperature compensation of the pressure sensor signal may be desirableand, in the case when a magnetostrictive sensor is employed, thetemperature compensation may for example be achieved by methods of thetype described in the present applicant's United States PatentApplication Publication No. 2007/0096724, the contents of which arehereby incorporated by reference.

When a pressure sensor is provided within a terminal cavity of aninjector, the electrical connection to the sensor may be separate fromor integrated with the electrical connection to the actuator. In onevariant, the pressure sensor is integrated with an actuator electricalconnector. In these ways, the electrical connections to the sensor areparticularly straightforward.

The injector body may comprise several individual components. Forexample, separate sections of the injector body may house the needle,the actuator, the coupling between the actuator and the needle, theelectrical connections, and so on. The sections may be clamped togetherby an outer sheath or housing. When the injector body comprises two ormore individual components or sections, it is conceivable that thepressure sensor may be responsive to the relative displacement of twocomponents, where the displacement is caused by and relates to thepressure of fuel within the supply passage extending through one or moreof the components.

It will be appreciated that the location of the sensor within theinjector body may be different from the locations described above.Indeed, the position of the sensor could be anywhere within the injectorbody, provided that the sensor is capable of sensing the strain,deformation or deflection in the injector body that results from thefuel pressure in the supply passage. In this way, the present inventioncan be employed in fuel injectors with various arrangements ofcomponents or which operate differently from those described above.

By providing fuel pressure sensors in each individual injector of anengine, the present invention allows and extends to continuousmonitoring and tracking of the fuel pressure that exists within the fuelinjector during operation of the injector. In this way, the pressure ofthe fuel when an injection event takes place can be accuratelydetermined, so that control of the quantity of fuel injected can beimproved in comparison to previous injector control systems.

Various methods for operating and controlling fuel injectors accordingto the invention in an internal combustion engine will now be described.

FIG. 7 shows a fuel injection system 150 according to the invention. Thefuel injection system 150 is similar to the conventional system shown inFIG. 1, except in that each of the fuel injectors 152 are provided withan integrated fuel pressure sensor as previously described, and the ECU154 receives signals from the pressure sensors in each of the fuelinjectors 152.

Those components of the fuel injection system 150 previously describedwith reference to FIG. 1 are indicated in FIG. 7 with reference numeralscorresponding to those of like components in FIG. 1.

To obtain a value for the pressure of fuel in the supply passage of aninjector 152, the output signal from that injector's pressure sensor isevaluated by the ECU 154. The ECU 154 includes a stored calibrationcurve, which relates the value of the signal from the pressure sensor tocorresponding values for the fuel pressure in the supply passage.

The calibration curve may, for example, be obtained by testing aninjector fitted with intrusive pressure sensors which monitor thepressure in the fuel supply passage directly.

An alternative method for obtaining a calibration curve is shown in FIG.8, and FIG. 9 shows apparatus suitable for performing the method of FIG.8. In this case, the sensor characteristic or calibration curve isinstead obtained by testing individual injectors 152 after theirmanufacture, for example by applying (at 400 in FIG. 8) a known fluidpressure to the supply passage by way of a fluid supply 412, measuring(402 in FIG. 8) an output value of the pressure sensor 158 of theinjector 152 and recording (404 in FIG. 8) the output value and thefluid pressure as a sensor characteristic, using a data recorder 414.The fluid pressure in the supply passage is then adjusted (408 in FIG.8) to another known value for measurement and recording of furtheroutput values of the pressure sensor 158. Once a predetermined number ofcalibration values have been recorded (406 in FIG. 8), the sensorcharacteristic is encoded (410 in FIG. 8) using an encoder 416.

FIG. 10 shows a method for programming an ECU of the fuel injectionsystem with the calibration information, and FIG. 11 is a schematicdiagram of apparatus suitable for performing the method of FIG. 10. Thecalibration curve and other sensor characteristics may optionally beencoded in a machine-readable format and supplied with the fuel injector152, for example as a two-dimensional barcode 418 printed on theinjector 152 as shown in FIG. 11, or on the packaging of the injector.The encoded data 418 may then be read by a reader 420 (at 500 in FIG.10) and decoded in a decoder 422 (502 in FIG. 10) to determine thesensor characteristics, including the calibration curve. The sensorcharacteristics are then input to the ECU 154 (504 in FIG. 10) duringassembly of the engine or upon replacement of an injector. Furthermore,other characteristics of the fuel injector, for example the actuatorbehaviour, may also be encoded in the barcode for input to the ECU 154.

Referring again to FIG. 7, the quantity of fuel delivered to acombustion chamber during an injection event is related to the pressureof fuel at the tip of the nozzle of the respective injector 152,adjacent to the orifices. Ideally, therefore, the pressure sensor islocated close to the tip of the nozzle. However, in the presentinvention it is often more convenient to locate the pressure sensorclose to the supply passage at a position remote from the nozzle tip. Inthat case, the ECU 154 applies a correction factor to the pressuresensor signal to evaluate the fuel pressure in the supply passage. Thecorrection factor may, for example, account for the dynamic pressurelosses in the supply passage between the vicinity of the pressure sensorand the nozzle tip.

During normal operation of the engine, the pressure sensors associatedwith each injector 152 provide their respective signals to the ECU 154.Each signal corresponds to the instantaneous local pressure in one ofthe injectors 152. When the signals are considered together, the timeaverage of these local pressures will vary slightly from one another,for example as a result of sensor drift, calibration errors, or offsetinaccuracies.

The ECU 154 can correct for such errors by an adaptive learning method,for example as shown in FIG. 12. At 600 in FIG. 12, the output from asensor is measured, and at 602 the average sensor output is determinedas an average local pressure. At 604, the ECU 154 compares the averagelocal pressure recorded in the injector 152 with a reference value, forexample the average local pressure recorded by the sensors in each ofthe other injectors 152. At 606 in FIG. 12, if the difference betweentwo local pressure values exceeds a pre-defined threshold value then, at608 in FIG. 12, the ECU 154 responds by applying a correction offset tothe sensor signal from the injector 152 from which the erroneous readingoriginates. The average local pressures are again compared. If thedifference still exceeds the threshold value, a further correctionoffset is applied, and this process repeats iteratively until thedifference falls below the threshold value. At that stage, thecorrection offset is stored in the ECU 154 and is applied thereafter tothe signal from the corresponding injector 152 for use in subsequentcalculations.

The instantaneous injection pressures will also differ from injector toinjector, as a result of hydraulic disturbances due to the injectionevents. For example, when a first injector performs an injection, thelocal pressure will drop rapidly in that injector. The pressure in asecond injector will decay at a slower rate, because the pressure droptakes some time to be transmitted from the first injector, through thefuel rail, and to the second injector.

In the embodiment shown in FIG. 7, no pressure sensor is provided in thefuel rail 156 of the system 150. Therefore, no port need be provided inthe fuel rail 156 for such a sensor. The mean pressure in the fuel rail156 can be estimated in the ECU 154 by calculating an appropriateaverage of the local pressure values from the individual injectors 152.In this way, the high-pressure fuel pump inlet metering valve 40 and thefuel rail pressure control valve 42 can be controlled by the ECU 154,using the estimated fuel rail pressure.

Should one of the pressure sensors associated with the injectors 152fail, the ECU 154 can detect the failure by checking for erroneoussignals, and can exclude that sensor from further calculations tocalculate the estimated rail fuel pressure. In this way, the fuelinjection system 150 can continue to function even if one or morepressure sensors fail.

When the injection strategy of the engine demands a single injectionevent per fuel injector 152 over one combustion cycle, it can be assumedthat the pressure perturbations generated by the previous injectionevent for that cylinder, and for any other cylinder, will have decayedbefore the next injection event begins. In this way, the local pressurein each injector prior to an injection event will be stable, and thetime for the forthcoming injection event can be accurately calculated bythe ECU 154 on the basis of the measured local pressure.

When the injection strategy of the engine demands more than oneinjection event per fuel injector per combustion cycle, pressureperturbations from a first injection event in the injection sequence maystill be present when a second injection event in the sequence takesplace. The present invention provides a method for correcting orcompensating for these pressure perturbations, so that the correctquantity of fuel is delivered into the combustion chamber by eachinjection event. A first expression of such a method will now bedescribed with reference to FIG. 12. Apparatus suitable for performingthe method is shown schematically in FIG. 14. The apparatus comprises anECU 154 and fuel injectors 152 each with integrated pressure sensors158, for example as shown in FIG. 7.

The hydraulic behaviour of the fuel injector 152—particularly the way inwhich pressure waves tend to propagate in a fuel injector 152—can becalculated or measured relatively accurately. Furthermore, the dynamicsof the pressure waves that arise as a result of injection events arepredictable over time when the hydraulic characteristics of the injector152 are known. Therefore, by predicting a local fuel pressurecharacteristic within an injector 152, such as the evolution with timeof a pressure wave within an injector 152, an expected local pressure atthe time of a subsequent injection event can be determined frompre-injection measurements of the local pressure.

FIG. 15 shows schematically the evolution of the local fuel pressure 160within a fuel injector 152 following an injection event. Thus FIG. 15could also represent the output signal of a fuel pressure sensor 158integrated in a fuel injector 152.

At 200 in FIG. 12, prior to an injection event the fuel pressure sensor158 of the corresponding injector 152 is interrogated repeatedly atpredetermined or otherwise known intervals. The optimum number andfrequency of the interrogations depends upon the hydrauliccharacteristics of the injector, but in a typical example at least teninterrogations are be performed at a frequency of 50 KHz or more. Theinterrogations, indicated at 162 a, 162 b and 162 c in FIG. 15, generatea sequence of local fuel pressure values 164 a, 164 b and 164 crespectively. Three interrogations 162 a-162 c are shown but there couldbe any desired number of interrogations. At 202 in FIG. 12, a processor166 of the ECU 154 compares the results 164 a-164 c of eachinterrogation 162 a-162 c to determine whether the pressure is stable.If the ECU 154 detects no significant difference between the results 164a-164 c of the interrogations 162 a-162 c, then no pressure wave ispresent. It can be assumed that the impending injection event will notbe affected by pressure perturbations and no correction to the injectiontime is applied.

If, however, the ECU 154 detects a difference between the results 164a-164 c of the interrogations 162 a-162 c, as shown in FIG. 15, then itis established that a pressure wave is present. A model of the hydraulicbehaviour of the injector 152 is retrieved from storage in a memory 168of the ECU 154, and, at 204 in FIG. 12, the results 164 a-164 c of theinterrogations 162 a-162 c are input to the model to predict how thepressure wave will evolve over the duration of the forthcoming injectionevent.

The model may, for example, comprise a number of stored sets of pressurewave characteristic data acting as common profiles, fingerprints orsignatures that show how the local pressure varies with time followinginjection events, taking account of other parameters such as fuelpressures and injection times. These data sets could be obtained bycalculation or during calibration procedures using test systems. Inoperation, the ECU 154 matches the results 164 a-164 c of theinterrogations 162 a-162 c to the stored data. Once a match is found,the data corresponding to the pressure wave can be retrieved from thememory 168 and analysed by the processor 166 to predict, at 206 in FIG.12, how the pressure wave will propagate over the forthcoming injectionevent.

If necessary, the ECU 154 determines and applies a correction to theinjection duration, at 208 in FIG. 12, so as to ensure that the desiredquantity of fuel is injected. For example, the hydraulic model mayprovide as an output a predicted mean value for the local pressure inthe injector 152 over the expected duration of the forthcoming injectionevent. This predicted mean value is then used to calculate the injectionduration required to inject the required quantity of fuel. Thisinjection duration may be greater than or less than the nominalinjection duration which would be necessary to deliver the required fuelhad the local pressure been determined as stable.

At 210 in FIG. 12, the required injection time is output to an injectorcontrol unit 170 of the ECU 154, which generates an injector controlsignal. The injector control signal is output to the actuator 172 of thefuel injector 152 to actuate the opening and closing movement of theneedle.

FIG. 16 shows a method for mitigating the effects of pressure sensorfailure. At 700 in FIG. 16, the ECU measures the output from a sensor158 and, at 702, the ECU checks the sensor output for errors oranomalies indicative of failure of the sensor 158, for example a zerooutput. If the sensor output is determined as erroneous (at 704), theECU excludes the signal from that pressure sensor 158 from furthercalculations and can instead use an alternative input for the hydraulicbehaviour model or other calculation. For example, should the pressuresensor 158 of one of the injectors 152 fail, the ECU 154 can apply theinjection times calculated for another one of the injectors 152undergoing the same or a similar injection sequence to the injector withthe failed sensor. In this way, a degree of robustness against sensorfailure is provided.

The method for correcting or compensating for pressure perturbations mayconceivably be applied to fuel injection systems provided with a railfuel pressure sensor, instead of or in addition to pressure sensorsintegrated in the injectors. Thus, a second expression of the method,using only a rail pressure sensor, will now be described with referenceto FIG. 17. A system suitable for performing the method of FIG. 17 isshown schematically in FIG. 18, which is similar to the apparatus ofFIG. 14 except in that a pressure sensor 174 is provided in the fuelrail 176, and in that the fuel injectors 178 do not include integratedpressure sensors. The ECU 180 receives input signals from the railpressure sensor 174.

At 300 in FIG. 17, prior to an injection event the rail pressure sensor174 is interrogated repeatedly to give a sequence of rail fuel pressurevalues. At 302 in FIG. 17, a processor 182 of the ECU 180 compares theresults of each interrogation to determine whether the pressure isstable. If the ECU 180 detects no significant difference between theresults of the interrogations, then it is assumed that the impendinginjection event will not be affected by pressure perturbations and nocorrection to the injection time is applied.

If, however, the ECU 180 detects a difference between the results of theinterrogations, then it is established that a pressure wave is presentin the fuel rail 176, which will affect the local fuel pressure withinthe injectors 178.

A model of the hydraulic behaviour of the fuel system, including thefuel injectors 174 and the fuel rail 176, is stored within a memory 184of the ECU 180. At 304 in FIG. 17, the results of the interrogations areinput to the fuel system hydraulic model to produce estimates of thelocal pressure within a fuel injector 178, corresponding to the measuredrail fuel pressure values.

In addition, a model of the hydraulic behaviour of each injector 178 isstored within the memory 184 of the ECU 180. At 306 in FIG. 17, theestimates of the local pressure within a fuel injector 178, calculatedfrom the rail pressure measurements at 304, are input to the injectorhydraulic model to provide, at 308 in FIG. 17, an output comprising aprediction of how the pressure wave will evolve in the fuel injectorover the duration of the forthcoming injection event.

If necessary, at 310 in FIG. 17 the ECU 180 applies a correction to theinjection duration so as to ensure that the desired quantity of fuel isinjected, as in the first embodiment of the method. At 312 in FIG. 17,the required injection time is output to an injector control unit 186 ofthe ECU 180, which generates an injector control signal. The injectorcontrol signal is output to the actuator 188 of a fuel injector 178 toactuate the opening and closing movement of the needle.

In a variant of the method of FIG. 17, the hydraulic behaviours of thefuel system and the fuel injectors are integrated into one model, sothat the measured rail fuel pressure values are input to the model, andthe output of the model is the predicted pressure wave evolution withinthe fuel injector.

It will be appreciated that the method is not limited by the location ofthe pressure sensor or sensors. For example, more than one rail pressuresensor may be provided, so as to generate a more accurate picture of thefuel pressure evolution within the rail for input to the model. Sensorsmay instead be connected with or provided within the jumper pipes whichconnect the respective injectors to the fuel rail. Sensors in two ormore different locations could be used in combination to provideinformation to help predict the fuel pressure evolution within theinjectors.

When the method utilises measurements from one or more sensors remotefrom the individual fuel injectors, such as in the second expression ofthe method, the fuel pressure evolution recorded by the sensor orsensors may result from a combination of pressure waves generated by thefuel injectors, the rail pressure control valve, the high-pressure fuelpump and so on. Therefore, the method may provide for the identificationof the contribution to the measured fuel pressure evolution ofindividual sources of pressure waves. For example, the sensor output maybe recorded during periods when no fuel injections take place, such asduring engine over-run. The recorded sensor output in these conditionsreflects only those pressure waves arising from components other thanthe fuel injectors. This ‘injection-free’ output can then be provided asan additional input to the hydraulic model, to allow more accuratedetermination of the predicted local fuel pressure at the injectors.

The hydraulic models may be implemented in the ECU as algorithms, aslook-up tables, or in other suitable forms. The models may be generatedusing calibration data obtained during testing or manufacture ofengines, or may be calculated using computational fluid dynamicstechniques.

1. A fuel injection system for an internal combustion engine, the systemcomprising: a plurality of fuel injectors; an accumulator arranged tosupply fuel to the fuel injectors in use; a pressure sensor formeasuring the pressure of fuel in the accumulator in use; and a controlunit arranged to receive a pressure signal from the pressure sensor toconstruct an hydraulic behavior profile; to predict, using the hydraulicbehaviour profile, the fuel pressure that will prevail in an injectorduring an injection event; and to supply control signals to that fuelinjector to control the amount of fuel injected during that injectionevent in accordance with the predicted fuel pressure.
 2. A fuelinjection system according to claim 1, wherein the control unitcomprises: a processor programmed repeatedly to sample the pressuresignal from the pressure sensor to construct the hydraulic behaviourprofile.
 3. A fuel injection system according to claim 1, wherein thecontrol unit comprises: a memory for storing an hydraulic behaviourmodel; and a processor programmed to apply the hydraulic behaviourprofile to the stored model to predict the fuel pressure that willprevail in the injectors during injection events.
 4. A method of fuelinjection, comprising: constructing an hydraulic behaviour profile byfuel pressure measurement; using the hydraulic behaviour profile topredict fuel pressure that will prevail in a fuel injector during aninjection event; and supplying a control signal to the fuel injector tocontrol the amount of fuel injected during the injection event inaccordance with the predicted fuel pressure.
 5. A method of fuelinjection according to claim 4, wherein the hydraulic behaviour profileis constructed by repeatedly sampling fuel pressure.
 6. A method of fuelinjection according to claim 4, comprising: retrieving a storedhydraulic behaviour model; and applying the hydraulic behaviour profileto the stored model to predict the fuel pressures that will prevail inthe fuel injector during the injection event.
 7. A method of fuelinjection according to claim 4, comprising constructing an hydraulicbehaviour profile by measuring fuel pressure in an accumulator arrangedto supply fuel to a plurality of fuel injectors in use; predicting,using the hydraulic behaviour profile, the fuel pressures that willprevail in the fuel injectors of the plurality during injection events;and controlling the fuel injectors of the plurality to control theamount of fuel injected during injection events in accordance with thepredicted fuel pressures.
 8. A computer program product comprising atleast one computer program software portion which, when executed in anexecution environment, is operable to implement the method of claim 4.9. A data storage medium having the or each computer program softwareportion of claim 8 stored thereon.
 10. A method of predicting a fuelpressure characteristic in a fuel injector of a fuel injection systemduring an injection event, the method comprising: measuring, before theinjection event, a fuel pressure characteristic within the fuelinjection system; and determining, using the measured fuel pressurecharacteristic, a predicted fuel pressure characteristic in the fuelinjector during the injection event.
 11. A method of predicting a fuelpressure characteristic according to claim 10, wherein the fuel pressurecharacteristic is measured by repeatedly interrogating a pressure sensorbefore the injection event.
 12. A method of predicting a fuel pressurecharacteristic according to claim 10, wherein the fuel pressurecharacteristic is measured within the fuel injector.
 13. A method ofpredicting a fuel pressure characteristic according to claim 10, whereinthe fuel pressure characteristic is measured at a location in the fuelinjection system upstream of the fuel injector.
 14. A method ofpredicting a fuel pressure characteristic according to claim 13, whereinthe fuel pressure characteristic is measured in an accumulator upstreamof the fuel injector.
 15. A method of predicting a fuel pressurecharacteristic according to claim 10, the method comprising inputtingthe measured fuel pressure characteristic to a model for hydraulicbehaviour to determine the predicted fuel pressure characteristic in thefuel injector during the injection event.
 16. A method of predicting afuel pressure characteristic according to claim 10, wherein the measuredfuel pressure characteristic comprises a sequence of fuel pressurevalues.
 17. A method of predicting a fuel pressure characteristicaccording to claim 10, wherein the predicted fuel pressurecharacteristic comprises a predicted average fuel pressure in the fuelinjector during the injection event.
 18. A computer program productcomprising at least one computer program software portion which, whenexecuted in an execution environment, is operable to implement themethod of claim
 10. 19. A data storage medium having the or eachcomputer program software portion of claim 18 stored thereon.
 20. Amethod of correcting a nominal fuel injection demand, comprising:predicting a fuel pressure characteristic in accordance with the methodof claim 10; calculating a correction factor based on the predicted fuelpressure characteristic during the injection event; and applying thecorrection factor to the nominal fuel injection demand to compensate forvariations in fuel pressure during the injection event.
 21. A method offuel injection, comprising: determining a nominal fuel injection demandaccording to engine operating parameters; and correcting the nominalfuel injection demand in accordance with the method of claim 20.